US Patent 7449633 Electrical box for concrete walls

An electrical outlet box for accommodating an electrical fixture includes a generally rectangular box having a back wall, a perimetrical side wall surrounding said back wall defining an open front face and a box interior, said side wall includes of a first and second set of generally parallel spaced apart wall portions; and a slidable extension nested within said generally rectangular box, that can be moved from within the generally rectangular box to an extended position beyond the open front face.